The present invention relates to a method of manufacturing an optical fibre including the following steps:                i) providing a hollow substrate tube,        ii) passing doped or undoped reactive, glass-forming gases through the interior of the hollow substrate tube,        iii) creating such conditions in the interior of the hollow substrate tube that deposition of glass layers onto the interior of the hollow substrate tube takes place, wherein a non-isothermal plasma is reciprocated between two reversal points along the substrate tube, wherein the velocity of movement of the plasma decreases to zero from each deceleration point to each reversal point,        iv) subjecting the substrate tube thus obtained to a collapsing treatment so as to form a solid preform, and        v) drawing an optical fibre from the solid preform.        
In addition to that, the present invention relates to a preform for manufacturing an optical fibre, as well as to an optical fibre.
Such a method is known per se from U.S. Pat. No. 7,522,800 in the name of the present applicant.
US 2008/0295541 relates to a method of manufacturing an optical fiber preform using a high frequency induction thermal plasma.
U.S. Pat. No. 5,318,612 relates to a method for producing a preform for optical fibers, comprising the steps of thermic glazing of internal and external surfaces of the bar, depositing a vitreous coating on the internal surface of the bar, said vitreous coating being provided for the subsequent formation of a core of the optical fibers, and collapsing the bar, various temperatures of the bar required for the glazing, depositing, and collapsing steps being obtained by varying the temperature of the oven.
Using the present invention, preforms for optical fibres are produced by means of an internal chemical vapour deposition technique (CVD), which process involves the deposition of doped or undoped reactive, glass-forming gases on the inside of a hollow substrate tube. Such reactive gases are supplied on one side of the substrate tube, viz. the entrance side, forming a glass layer on the interior of the substrate tube under the influence of special process conditions. A reaction zone is reciprocated along the substrate tube so as to form a glass layer. The reaction zone, in particular a plasma generator, supplies high-frequency energy, as a result of which a plasma is generated in the interior of the substrate tube, under which plasma conditions the reactive, glass-forming gases will react (the plasma CVD technique). It is also possible, however to supply the energy by means of heat, in particular by using burners, on the outer side of the substrate tube or via a furnace that surrounds the substrate tube. A common feature of the aforesaid techniques is the fact that the reaction zone is reciprocated with respect to the substrate tube.
According to a Plasma Chemical Vapor Deposition (PCVD) process the quartz substrate tube is placed in a resonant cavity along its cylindrical axis, and a mixture of raw material gases containing for example, O2, SiCl4, and one or more dopants, e.g. GeCl4, C2F6 is passed into the tube; a local plasma is simultaneously produced within the resonant cavity, causing the reaction of Si, Ge, O, etc., thereby SiOx mainly doped with Ge/F is formed by direct deposition on the inside surface in the substrate tube, forming a core layer and one or more cladding layers. The hollow quartz glass substrate tube is surrounded by a furnace. During the internal deposition step the resonant cavity moves along the cylindrical axis of the substrate tube as to uniformly coat the whole length of the substrate tube with glass layers. When deposition is finished the substrate tube is collapsed into a solid core rod, which has a SiO2 core layer doped with dopants like F, Ge and a surrounding SiO2 cladding undoped or doped with F—Ge. Such a core rod is made into an optical fiber preform, for example by inserting into a jacket tube or by OVD overcladding, which can be drawn into optical fiber used as the transmission medium for communication.
High temperatures may lead to the inclusion of the undesirable OH groups in the external surface of the support tube. Since the support tube forms part of the fibre yet to be drawn, it is to be expected that said included OH groups will present problems as regards the optical properties of the glass fibre that is eventually obtained. The OH groups that are included on the outer side can diffuse inwardly, that is, in the direction of the core, in the course of the further processing steps, when temperatures are high. It has become apparent that said OH groups produce adverse effects in the light conducting part of the optical fibre. After all, the OH groups exhibit a wide absorption peak at 1385 nm. As a result, additional signal loss occurs in the optical glass fibre with the transmission wavelengths around 1300 nm and 1500 nm that are currently being used. Moreover, said absorption peak at 1385 nm limits the use of the fibre over a large wavelength range that recent developments require. Thus it is desirable that the effect of the inwardly diffusing OH groups is minimized, thus minimizing the signal loss at the standard transmission wavelengths, which makes the fibre very suitable from a commercial point of view.
A method for making a preform which is substantially free of OH impurities is known from U.S. Pat. No. 5,397,372.
US 2005/0000253 relates to a method for manufacturing low water peak single mode optical fibers by PCVD technology, wherein the attenuation of the single mode optical fiber produced thereby at 1383 nm being lower than the specified value at 1310 nm. Said US patent application focuses on the content of impurities in the gas mixture, the hydroxyl content of the jacket tube and the relative humidity of the environment during the deposition process.
U.S. Pat. No. 7,519,256 relates to a method for manufacturing an optical preform wherein the velocity of the reaction zone for the deposition of the inner cladding is set so that the acceleration of the reaction zone near the point of reversal at the supply side for depositing the inner cladding is higher than the acceleration of the reaction zone near the point of reversal at the supply side for depositing the outer cladding.
U.S. Pat. No. 4,741,747 relates to a method of manufacturing optical fibres, wherein the so-called end taper is reduced by moving the plasma nonlinearly as a function of time in the region of the reversal point and/or by varying the intensity of the plasma along the length the glass tube.
U.S. Pat. No. 4,857,091 relates to a method of manufacturing optical fibres, in which a number of parameters are mentioned that influence the axial position of the local deposition zone with respect to the plasma generator, which parameters include:                (i) periodically varying the microwave power,        (ii) periodically varying the pressure in the substrate tube, and        (iii) periodically varying the stroke velocity of the resonator being reciprocated over the tube.        
European patent application No. 0 038 982 relates to a method of manufacturing optical fibres wherein the plasma generator is moved along the substrate tube, which plasma generator produces a hot zone such that the hot zone can be considered as a so-called “tandem hot zone” which includes at least two zones, viz. zone I and zone II. Although it is mentioned in said document that deposition rates or deposition compositions can be changed so as to prevent the occurrence of so-called taper ends, it is not indicated in said document what specific operations such a treatment involves.
European patent application No. 0 333 580 relates to a device for manufacturing preforms for optical fibres in which a variable power microwave generator is used, in which no use is made of a non-isothermal plasma which is reciprocated between two reversal points along the substrate tube, however.
From British patent publication GB 2 118 165 there is known a method of manufacturing an optical fibre in which the velocity of the heat source being moved axially along the substrate tube is in accordance with a specific mathematical equation, wherein the velocity of said heat source along the tube is a function of the position of said heat source along the substrate tube, so that the total deposition thickness of the glass layers is substantially constant over the length of the tube.
From U.S. Pat. No. 5,188,648 to the present applicants there is known a method of manufacturing optical fibres wherein the movement of the plasma is interrupted each time the plasma reaches the reversal point near the gas entry point of the substrate tube, whilst the glass deposition continues, wherein the period during which the plasma movement is interrupted is at least 0.1 second.